The present invention relates to a noise elimination circuit for eliminating noise in an audio signal in a magnetic tape recording and reproducing apparatus for recording and reproducing a video signal, and more particularly to a noise elimination circuit for eliminating noise in an audio signal in a video tape recorder/reproducer which simultaneously records a video signal and the audio signal in a common record track in a frequency-multiplexed form and reproduces it.
In a conventional magnetic recording and reproducing apparatus which records a television signal derived by receiving a television broadcasting wave or a television signal derived from a television camera on a magnetic tape and reproduces it therefrom, a video signal and an audio signal are usually recorded on separate record tracks. More particularly, the video signal and the audio signal contained in the television signal are separated from each other and the video signal is used to modulate a carrier signal and the modulated signal is recorded on the magnetic tape by one or more rotary magnetic head (video head). The record tracks for the video signal recorded on the magnetic tape are orthogonal or oblique to the direction of travel of the magnetic tape so that a higher relative speed between the magnetic tape and the magnetic head is attained. On the other hand, the audio signal is recorded on an upper edge or a lower edge of the magnetic tape along the direction of travel of the magnetic tape by a stationary head (audio head).
In such a magnetic recording and reproducing apparatus, in order to record a long time television signal on a given length of magnetic tape, the width of the record tracks for the video signal is shortened and the speed of the magnetic tape is decreased so that the space between adjacent video tracks is shortened as much as possible. When the speed of the magnetic tape is reduced such that the adjacent video tracks almost contact each other, a high signal-to-noise ratio (S/N) of the recorded video signal can be maintained because the video head for recording the video signal records the video signal on the magnetic tape while it is rotating and hence the relative speed between the magnetic tape and the video head is sufficiently high.
However, since the audio signal of the television signal is recorded in a zero modulation condition on the magnetic tape along the direction of travel of the magnetic tape as is done in a conventional audio tape recorder, high frequency components of the audio signal are not recorded when the speed of the magnetic tape is reduced and the quality of the recorded audio signal is degraded to make it inapplicable to practical use.
It has been proposed therefore to record the audio signal and the video signal together on the video tracks by the video head. In such a recording system, an audio carrier signal of 1 MHz, for example, is frequency modulated by the audio signal and the resulting modulated signal is allotted to a frequency band which is lower than a frequency band of the video signal, and those two signals are simultaneously recorded on the video tracks of the magnetic tape by the video head. According to this recording system, since the relative speed between the video head and the magnetic tape is high, for example, approximately 4 m/sec, the audio signal can be recorded with a sufficiently high signal level over an entire range of the audio frequency band.
In general, when the video signal is recorded on the magnetic tape orthogonally or obliquely to the direction of travel of the magnetic tape, one single field of the video signal is recorded on a track of the magnetic tape extending from the upper end to the lower end or from the lower end to the upper end and the next single field of the video signal is recorded on the next adjacent video track. Thus, when the recorded signal is reproduced by the video head, the reproduced video signal is discontinuous track by track and not continuous.
When the frequency-multiplexed video signal and the audio signal are recorded on the video tracks, the audio signal reproduced is also discontinuous track by track as is the video signal. An affect by the discontinuity of the video signal does not appear on the picture tube of the television receiver because the video signal is recorded on the respective video tracks field by field and the switching from one video track to the next occurs during the flyback period of the television signal. However, when the audio signal is discontinuous, it causes noise which is reproduced by the speaker.
The video signal recorded on the video tracks also contains a horizontal synchronizing signal of the television signal. Thus, when the video signal and the audio signal are recorded on the same track, a periodic noise related to the horizontal synchronizing signal is created in the audio signal due to the nonlinearity of a magnetic circuit including the magnetic tape and the video head. Accordingly, it is necessary to eliminate such noise.